Polymers, including homopolymers and copolymers, which are both biocompatible and absorbable in vivo are well known in the art. Such polymers are typically used to manufacture medical devices which are implanted in body tissue and absorb over time. Examples of such medical devices manufactured from these absorbable biocompatible polymers include suture anchors, sutures, staples, surgical tacks, clips, plates and screws, etc.
Absorbable, biocompatible polymers useful for manufacturing medical devices include both natural and synthetic polymers. Natural polymers include cat gut, cellulose derivatives, collagen, etc. Synthetic polymers include aliphatic polyesters, polyanhydrides, poly(ortho)esters, and the like. Natural polymers typically absorb by an enzymatic degradation process in the body, while synthetic absorbable polymers typically degrade by a hydrolytic mechanism.
Synthetic absorbable polymers which are typically used to manufacture medical devices include homopolymers such as poly(glycolide), poly(lactide), poly(.epsilon.-caprolactone), poly(trimethylene carbonate) and poly(p-dioxanone) and copolymers such as poly(lactide-co-glycolide), Poly(.epsilon.-caprolactone-co-glycolide), and poly(glycolide-cotrimethylene carbonate). The polymers may be statistically random copolymers, segmented copolymers, block copolymers, or graft copolymers. It is also known that both homopolymers and copolymers can be used to prepare blends.
There is a constant need in this art for new polymer compositions having improved physical properties when molded or extruded into medical devices and further having excellent in vivo properties. For example, it is known that poly(lactide) and many copolymers of lactide and glycolide rich in lactide repeating units have superior in vivo properties. However, molded articles manufactured from these copolymers are known to have poor dimensional stability due to a lack of crystallinity.
Additionally, for certain applications, such as plate and screw fixation devices, it is necessary to be able to bend the device and then retain the shape of the device to the contours of a body structure.
Furthermore, such devices should have excellent palpability. That is, the device, in vivo, should be able to soften and dissolve away slowly upon absorption, rather than fragmenting into small granules which can cause tissue reaction.
Accordingly, what is needed in this art are novel polymer mixtures which have improved dimensional stability, shape retention, and palpability, while retaining the excellent strength, stiffness and breaking strength retention (BSR) found in poly(lactide) homopolymers and poly(lactide-co-glycolide) copolymers. Breaking strength retention is a conventionally known standard method of measuring the strength of a device made of a bioabsorbable polymer as a function of time under biological conditions in vitro or as a function of time after being implanted in vivo.
As described in U.S. Pat. Nos. 5,080,665 and 5,320,624 and Canadian applications 2,079,274 and 2,079,275, various ductile, bioabsorbable polymers (e.g., poly(.epsilon.-caproactone), poly(trimethylene carbonate), and poly(p-dioxanone)) have been blended with amorphous or low crystallinity poly(lactide) hompolymers and poly(lactide-co-glycolide) copolymers to improve device bendability at room temperature and their resistance to stress cracking. By the addition of these ductile polymers to the blend, the stiffness of the material decreases to a point where it is possible to bend the device. Upon bending, crazes form, creating voids in the device which lead to permanent deformation. However, because of void formation, local stress concentrations may form, which can often lead to an unsatisfactory decrease in stiffness, strength and BSR.
Therefore, to this end, it would be highly desirable to develop blends which were not dependent upon large additions of a ductile polymer to create bendability in the fixation device, but were dependent upon smaller additions of a low melting polymer, in which the molded or extruded device when heated above the melting point of this low melting polymer could be bent or shaped to the contours of the fracture that is to be fixated. Once shaped, the device could be cooled to room or body temperature, recrystallizing the low melting polymer of the blend, and thereby, locking the newly formed shape of the device in place.
Additionally, since only small amounts of this low melting polymer or copolymer would be incorporated into the blend, the excellent strength, stiffness and BSR of the major phase of the blend, poly(lactide) hompolymer or poly(lactide-co-glycolide) copolymer, would be retained.